利用雷射粒徑探針分析淡海水混合時的懸浮顆粒分佈狀態

Abstract

河口環境因為為一淡海水交會處，於潮汐時段鹽度變化強烈，水體的電解質濃度和離子強度因而改變，進而產生明顯的物理化學反應。這些反應包含膠體或是顆粒態的凝絮作用(flocculation)、吸附作用(adsorption) 與脫附作用(desorption)，而這些作用被視為是水體中顆粒形成的重要一環。顆粒傳輸過程中加上重力牽引，這些載體最後終將沉積掩埋中。為了瞭解水體懸浮顆粒於淡海水交會混合時的特性，本研究使用了雷射粒徑探針於實驗室模擬測量，後再定點(in-situ)置於淡水河大稻埕碼頭，量測潮汐週期下的顆粒變化，並配合多層次網目過濾器取得0.4~10μm、10~63μm、63~153μm，等不同大粒徑大小之水體懸浮顆粒，比對光學粒徑與過濾後懸浮顆粒的總數粒徑，探討懸浮顆粒在總數與粒徑大小分佈現象。 過篩後的沉積物與AGMP-1人工顆粒，擁有一致性的分佈，不因鹽度的改變而產生濃度與顆粒分佈上的變化，而Chelex-100則受鹽度變化而改變其粒徑大小。淡水河現場觀測的部份，由退潮開始至漲潮後顆粒的分佈不僅僅受到鹽度的改變而有所波動，整體粒徑族群的分佈也跟著改變，第一部份是上午七點漲潮開始後的兩小時內，主要粒徑族群的分佈往 >74.5μm至460μm的粒徑上偏移，對比由CATNET過濾所得到的TSM發現 ≥63μm以上的顆粒不僅少於光學觀測顆粒，且與LISST-100X所判讀到的結果相反。判斷較大粒徑族群的顆粒為水中凝絮作用所生成，傳統的過濾方法後可能並不是適用，對於了解水中的顆粒生成與分佈而言，過濾採集顆粒並非是一種可靠的方式。

In nature water, transfer and transformations of suspended particles were controlled by chemical and physical processes such as flocculation, adsorption and desorption. Most suspended particles in the estuarine and coastal environments are in the form of flocs because of flocculation processes. Many of these flocs might be break up easily during sampling and filtering analysis, which cause the estimation error of suspended sediment concentration (SSC). In this work, series of laboratory and field experiments using LISST-100X in-situ particle size analyzer and sized fractionate filtration method (CATNET: 0.4μm~10μm, 10~63μm, 63~153μm, >153μm) to monitor the distributions and changes of suspended particles during tidal mixing. Laboratory experiment shows that the artificial particle (chelex-100) moved to smaller particle size under high salinity condition. And for field experiments in Danshui river, the measured volume concentration by LISST-100X of larger particles increased during flood tide, but samples of large size particles collected by vacuum filtration and CATNET shows a inverse results. This phenomenon confirms that, flocs and aggregates were fragile, the traditional vacuum filtration of sampling cause the breakage of flocs and leads to the wrongful estimation of particle size distribution and variation.